Contact lenses are widely used for correcting many different types of vision deficiencies. These include defects such as near-sightedness and far-sightedness (myopia and hypermetropia, respectively), astigmatism, and defects in near range vision usually associated with aging (presbyopia). Each type of defect requires a specific correction and coordinating manufacturing process or processes.
Astigmatism occurs because the refractive error in an eye is dependent upon spatial distribution of the optical error. Astigmatism is typically caused by one or more refractive surfaces, most commonly the anterior cornea, having a toroidal shape. Astigmatism can be corrected with an astigmatic ophthalmic lens, which usually has one spherical surface and one toroidal (cylindrical) surface.
Presbyopia occurs as a person ages because the lens of the eye loses its elasticity, eventually resulting in loss of the ability to focus at near distances. To compensate for presbyopia, ophthalmic lenses are required to be more positively powered or less negatively powered than the distance correction. Some presbyopic persons have both near vision and distance vision defects, requiring simultaneous or alternating vision lenses to properly correct their vision.
Simultaneous vision lenses refer to the class of bifocal or multifocal ophthalmic lenses in which optical power for distance vision and near vision are positioned simultaneously within the pupil area of a user's eye. They are generally composed of two or more concentric annular zones which alternately provide the distance and near power, or a multifocal zone having an aspheric surface which provides a continuous gradient of optical power over a selected range of powers.
Alternating vision refers to the class of segmented (translating) bifocal ophthalmic lenses in which the lens is divided into two or more optical zones. Typically the superior (upper) zone is for distance vision correction, whereas the lower zone is for near vision correction. The distance portion subtends the pupil of the eye in primary gaze. In downward gaze the add power or near portion (lower zone) of the lens subtends the pupil. Lenses for this type of defect can be created, for example, by molding, casting or lathing processes.
Additionally, some lens-wearers may need more than one correction. For example, a person with presbyopia may also have an astigmatism vision error. Those presbyopes may require ophthalmic lenses capable of correcting both astigmatism and presbyopia. Lenses that incorporate corrections for both types of defects usually combine one or more manufacturing processes or entail a lengthier single process.
Lenses that are designed to correct the above-referenced defects may be created through molding, casting or lathe-cutting. For example, contact lenses that are manufactured in large numbers are typically produced by a mold process. In those processes, the lenses are manufactured between two molds without subsequent machining of the surfaces or edges. Such mold processes are described, for example in U.S. Pat. No. 6,113,817, which is expressly incorporated by reference as if fully set forth herein. As such, the geometry of the lens is determined by the geometry of the mold. In a typical molding system, lenses are cycled through a series of stations on a semi-continuous basis. The cyclic portion of lens production generally involves dispensing a liquid crosslinkable and/or polymerizable material into a female mold half, mating a male mold half to the female mold half, irradiating to crosslink and/or polymerize, separating the mold halves and removing the lens, packaging the lens, cleaning the mold halves and returning the mold halves to the dispensing position. The polymerization of the material is determined by the application time, position, and amount of UV light applied. Similar to mold geometry, the UV radiation is generally altered for different types of lenses. As such, producing different types of lenses and powers may not be efficient.
For defocus or correction lenses there is typically one design parameter, which is the spherical power. Each different lens power requires at least one set of molding tools and/or molds. Hence, to provide a lens line serving most optical powers, a moderate number of molding tools and/or molds are needed. For toric lenses, at least three parameters must be considered: spherical power, cylindrical power, and the orientation of the cylindrical power. The permutations of all of these powers produce a large, almost unmanageable number of unique lens stock keeping units (SKUs), and an even larger number of molding tools and molds. Similarly, for multifocal lenses, a huge number of molding tools and molds is required.
Additionally, some persons require made-to-order (MTO) or customized lenses. Each customized lens required its own molding tools and molds. As such, the cost of MTO lenses is very high and may even be cost-prohibitive.